Regarding conventional molten metal plating methods for steel tubes, a typical example thereof is hot dip plating. For instance, in the zinc hot dip galvanizing method, where both inner and outer surfaces of a steel tube are galvanized by dipping said steel tube in a molten zinc bath as presented in Japanese patent application No. 116864/81, the steel tube is tilted 40 degrees or more to the horizontal immediately after being removed from the molten zinc bath in order to remove the surplus zinc adhering to the steel tube inner surface. Next, the surface is quickly heated above the melting point of zinc, and the surplus zinc adhering to the outer surface of the steel tube is removed by blowing compressed gas against the surface of the tube which bears the molten zinc.
To meet changing industrial requirements, steel manufacturers began making surface-treated steel sheet by plating cold and hot rolled steel sheet. This resulted in plated steel tubes being manufactured from surface-treated steel strip.
Because the steel tube made from this plated steel sheet ("steel sheet" hereinafter includes "steel strip") is welded after undergoing plating, plating on the weld surface peels off and it can then be repaired. However, problems associated with rust occurrence and rust preventive capability in galvanizing occur with this method.
Steel tubes with a metal-plated outer surface and a coated inner surface are used from a cost viewpoint, and represent one method of manufacturing molten metal plated steel tubes in a continuous manufacturing line.
One manufacturing process of this method of manufacturing molten metal plated steel tubes already developed by this inventor is disclosed in U.S. Pat. No. 3,927,816. That invention shows a series of process steps for manufacturing steel tubes with a molten metal plated outer surface, where steel strip is cold-formed into tubular form, welded, and finally given a continuous molten metal plating treatment.
On the other hand, in recent years, with the expansion of industrial applications and increased severity of environmental conditions, good corrosion resistance of the inner surfaces of steel tubes has become necessary along with increased requirements for metal plating of the inner surfaces. Under these circumstances, a technique that permits continuous metal plating of both the inner and outer surfaces of steel tubes formed from steel plate has not yet been perfected.
One current technique which can be cited is single-surface plating of steel sheets. Regarding this single-surface molten metal plating technique which is applicable for steel sheets and steel strip, the following typical methods are known. The first is the application of certain chemicals on one side of the sheet to prevent a reaction between the steel sheet and the molten metal, followed by immersion of the steel plate in a plating bath where a single surface is plated. The second is the application of plating on both surfaces and the removal of the plating layer on one surface mechanically or chemically. Zinc-plating of a single surface by roll coater constitutes the third method. However, these methods all aim at single-surface molten metal plating for steel sheet and steel strip.
Japanese patent application No. 116884/81 presents a conventional method of applying zinc plating to both inner and outer surfaces of steel tubes by dipping them in a molten zinc bath. However, this method is applicable for hot dip galvanizing of manufactured steel tubes cut to a certain length and has the problem of not being applicable to double-surface molten metal plating in a continuous production line where steel strip is cold-formed into tubular form, welded, then treated by molten metal plating.
U.S. Pat. No. 3,927,816 teaches that the desired effect can be obtained on the exterior of the steel tube, however, that patent does not teach molten metal plating of the inner surface of the tubular steel.
On the other hand, single-surface molten metal plating for steel sheet is not always applicable in a continuous steel tube production line, and involves numerous manufacturing problems.